The present invention relates to a regulated resonant converter. In particular to a series resonant converter for providing a direct current (DC) power supply to electromagnets.
Regulated resonant converters are well known and are used to convert an alternating current (AC) mains supply to a regulated source of electrical energy. Regulated resonant converters are widely used for various applications, for example battery chargers, induction heating and power supply to electromagnets. The following discussion is especially, although not exclusively, concerned with the use of resonant power converters in magnetic resonant imaging (MRI) systems, for providing controlled electrical energy to gradient coils for the purpose of modifying the magnetic field of an MRI magnet as required for imaging. Each is resonant converter functions as a DC electricity supply for a three-axis gradient amplifier whose output is in turn applied to the gradient coils. UK Patent Application GB-A-2311387 discloses a regulated resonant converter in an MRI system and is included herein by reference.
Known regulated resonant converters comprise a series resonant circuit through which current is switched alternately in opposite directions at a frequency which corresponds to, or which is close to, the resonant frequency of the series resonant circuit, by an arrangement of switching transistors fed via a rectifier from an AC mains supply. In this way, higher frequency perturbations are strongly attenuated. Operation of the switching transistors is controlled by signals generated in a control circuit in dependence upon a fed-back sample of an output voltage from the converter which is developed in the resonant circuit, and a crossover voltage derived in dependence upon current reversal in an inductor, which forms a part of the resonant circuit, regulation being effected in dependence upon modification of the fed-back sample.
A typical parallel loaded series resonant converter for an MRI system converts the input from a 50 Hz three-phase source of 400 V AC (or a 60 Hz source at 480 V AC) into six isolated outputs at 400 V DC. The load applied in parallel across a capacitor component of the resonant circuit can vary from zero to 25 kW.
The transistor switches used are typically insulated gate bipolar transistors (IGBT) but may also be power transistors or a gate turn-off thyristors.
Conventionally, the output voltage is regulated by controlling the switching frequency in a bridge arrangement of transistors, for example a half-bridge, having two switches, or an H-bridge, which has four switches. At peak line voltage and minimum load, the peak current in the switches and in the resonant inductor is at a maximum and power is lost as a result. The bridge arrangement and the resonant circuit are stressed more in the absence of a load than at full load.
It is therefore an object of the invention to obviate or at least mitigate the aforementioned problems.
In accordance with one aspect of the present invention, there is provided a method for regulating the output voltage of a resonant power converter, including the steps of:
a) providing current to a resonant circuit through a switching arrangement having a plurality of switches, the switches governing an alternating flow of current through the resonant circuit;
b) providing a feedback signal; and
c) controlling the behaviour of the switches in accordance with the feedback signal in order to vary the duration over which current passes through the resonant circuit.
The method preferably further includes the steps of:
d) detecting a current reversal event; and
e) generating a crossover signal corresponding to the current reversal event.
Advantageously, step c) corresponds to varying the voltage drop across the resonant circuit.
It is preferred that the switching arrangement comprises an H-bridge including: a first switch, a second switch, a third switch and a fourth switch; the first switch being disposed in series with the third switch and the second switch being disposed in series with the fourth switch, a first junction being provided between the first switch and the third switch and a second junction being provided between the second switch and the fourth switch, and the resonant circuit being disposed between the first junction and the second junction.
Step c) may comprise the generation of a repeating cycle of switching signals.
Preferably, the generation of a repeating cycle comprises the substeps of: providing the first switch with a first switching signal having a first ON-time of fixed duration, a first onset time, t0 and a first end time, t2; providing the second switch with a second switching signal having a second ON-time of fixed duration, a second onset time, t3 and a second end time, t5; providing the third switch with a third switching signal having a third ON-time of variable duration, the third ON-time beginning at the second onset time, t3 and ending at a third end time, t4; and providing the fourth switch with a fourth switching signal having a fourth ON-time of variable duration, the fourth ON-time beginning at the first onset time, t0 and ending at a fourth end time, t1.
The first ON-time and the second ON-time may have substantially the same fixed duration.
The third ON-time and the fourth ON-time may also have substantially the same variable duration within each repeating cycle of switching pulses.
Step c) can be achieved by varying the third end time, t4, of the third witching signal and the fourth end time, t1, of the fourth switching signal in accordance with the feedback signal.
In accordance with a further aspect of the present invention, there is provided a power converter apparatus for supplying a regulated output voltage, including: a resonant circuit; a switching arrangement having a plurality of switches and governing the direction and duration of flow of current through the resonant circuit; a feedback means for supplying a feedback signal; and a control circuit which controls the behaviour of the switches in the switching arrangement in accordance with the feedback signal; characterised in that the control circuit controls the switching arrangement by varying the duration over which current passes through the resonant circuit in accordance with the feedback signal.
The apparatus advantageously further includes a crossover detection means for detecting current reversal events in the resonant circuit and generating a crossover signal corresponding to detected current reversal events, the control circuit using the crossover signal to transmit a corresponding switching signal to at least one of the switches in the switching arrangement.
Preferably, the crossover detection means includes a current transformer comprising a secondary circuit coupled to the resonant circuit and each current reversal event in the resonant circuit induces a corresponding current reversal event in the secondary circuit.
It is preferred that the resonant circuit includes a capacitor and a first inductor in series, the first inductor connected to a first side of the capacitor.
The resonant circuit may further include a second inductor connected in series to the side of the capacitor opposite to the first side.
Advantageously, the feedback signal corresponds to an output voltage measured across the capacitor.
The switching arrangement preferably comprises an H-bridge arrangement.
The H-bridge may include a first switch, a second switch, a third switch and a fourth switch, the first switch being disposed in series via a first junction with the third switch, the second switch being disposed in series via a second junction with the fourth switch and the resonant circuit being disposed between the first junction and the second junction.
Advantageously, the control circuit controls the H-bridge by addressing each of the four switches independently with a corresponding switching signal.
It is preferred that a first switching signal addressing the first switch and a second switching signal addressing the second switch both have a fixed duration TF.
Likewise a third switching signal addressing the third switch and a fourth switching signal addressing the fourth switch preferably both have a duration TV that varies in accordance with the feedback signal.
The voltage, VC, across the resonant capacitor, CR, is a function not only of the switching frequency but also of the load and the voltage applied across the resonant circuit (VA-VB). Therefore, by controlling the voltage applied to the resonant circuit (VA-VB) rather than the frequency, the peak current at no load can be reduced to a level lower than the peak current at full load. Variation in the ON-times of the switching arrangement provides the control circuit with the necessary control over the level of voltage drop (VA-VB).